Super fine granular steel pipe and method for producing the same

ABSTRACT

A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (μm), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac 3  transformation point, and applying reducing at an average rolling temperature of θm (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac 3  transformation point, with di, θm and Tred being in a relation satisfying a prescribed equation.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a steel pipe containingsuper-fine crystal grains, which has excellent strength, toughness andductility and superior collision impact resistance and a method forproducing the same.

BACKGROUND ART

[0002] The strength of steel materials have been increased heretofore byadding alloying elements such as Mn and Si, and by utilizing, forinstance, controlled rolling, controlled cooling, thermal treatmentssuch as quenching and tempering, or by adding precipitation hardeningelements such as Nb and V. In the case of a steel material, however, notonly strength but also high ductility and toughness are required. Hence,a steel material with balanced strength and ductility as well astoughness has been demanded.

[0003] The reduction in crystal size is important in that it is one ofthe few means for increasing not only strength, but also both ofductility and toughness at the same time. Crystal grains sufficientlyreduced in size can be realized by, for example, a method whichcomprises preventing coarsening of austenite grains and obtaining fineferritic crystal grains from fine austenite grains by utilizing theaustenite—ferrite transformation; a method which comprises obtainingfine ferrite grains from fine austenite grains realized by working; or amethod which comprises utilizing martensite or lower bainite resultingfrom quenching and tempering.

[0004] In particular, controlled rolling comprising intense working inthe austenitic region and reducing size of ferrite grains by using thesubsequent austenite—ferrite transformation is widely utilized for theproduction of steel materials. Furthermore, a method for furtherreducing the size of ferrite grains by adding a trace amount of Nb andthereby suppressing the recrystallization of austenite grains is alsoknown in the art. By working in a temperature in the non-recrystallizingtemperature region, austenite grains grow as to form a transgranulardeformation band, and ferrite grains generate from the deformation bandas to further reduce the size of the ferrite grains. Furthermore,controlled cooling which comprises cooling during or after working isalso employed.

[0005] However, the fine grains available by the methods above havelower limits in the grain size of about 4 to 5 μm. Furthermore, themethods are too complicated to be applied to the production of steelpipes. In the light of such circumstances, a method comprising simpleprocess steps and yet capable of further reducing the grain size offerrite crystals for improving the toughness and ductility of steelpipes has been required. Moreover, concerning the recent increasingdemand for steel pipes having superior collision impact resistances toachieve the object of improving safety of automobiles, limits in cuttingcost has been found so long as the methods enumerated above areemployed, because they required considerable modification in processsteps inclusive of replacing the equipment and the like.

[0006] Furthermore, the improvement in resistances against sulfidestress corrosion cracks of steel pipes for use in line pipes, atpresent, hardness control is performed to lower the concentration ofimpurities and control the concentration of alloy elements.

[0007] Conventionally, fatigue resistance has been improved by employingthermal treatments such as quench hardening and tempering, inductionhardening, and carburizing, or by adding expensive alloy elements suchas Ni, Cr, Mo, etc. in large amounts. However, these methods hasproblems of impairing the weldability, and furthermore, of increasingthe cost.

[0008] A high strength steel pipe having a tensile strength of over 600MPa is produced by using a carbon-rich material containing carbon (C) ata concentration of 0.30% or more, or by a material containing C at ahigh concentration and other alloy elements added at large quantities.In the case of high strength steel pipes thus increased in strength bymethods above, however, the elongation properties tend to be impaired.Thus, in general, the application of intense working is avoided; in caseintense working is necessary, intermediate annealing is performed duringworking, and further thermal treatments such as normalizing, quenchingand tempering, etc., is applied. However, the application of additionalthermal treatment such as intermediate annealing makes the processcomplicated.

[0009] In the light of the circumstances above, a method which allowsintense working of high strength steel pipe without applyingintermediate annealing is demanded, and also, further reduction incrystal grains is desired for the improvement in workability of highstrength steel pipes.

[0010] An object of the present invention is to advantageously solve theproblems above, and to provide a steel pipe improved in ductility andcollision impact resistance without incorporating considerable change inproduction process. Another object of the present invention is toprovide a method for producing the same steel. Further, another objectof the present invention is to provide a steel pipe and a method forproducing the same, said steel pipe containing super fine grains havingexcellent toughness and ductility which are ferrite grains 3 μm or lessin size, preferably, 0.2 μm, and more preferably, 1 μm or less in size.

[0011] A still another object of the present invention is to provide ahigh strength steel pipe containing superfine crystal grains, which isimproved in workability and having a tensile strength of 600 MPa ormore, and to a method for producing the same.

DISCLOSURE OF THE INVENTION

[0012] The present inventors extensively and intensively performedstudies on a method of producing high strength steel pipes havingexcellent ductility, yet at a high production speed. As a result, it hasbeen found that a highly ductile high strength steel pipe havingwell-balanced strength and ductility properties can be produced byapplying reducing to a steel pipe having a specified composition in atemperature range of ferrite recovery or recrystallization.

[0013] First, the experimental results from which the present inventionis derived are described below.

[0014] A seam welded steel pipe. (φ42.7 mm D×2.9 mm t) having acomposition of 0.09 wt % C-0.40 wt % Si-0.80 wt % Mn-0.04 wt % Al washeated to each of the temperatures in a range of from 750 to 550° C.,and reducing was performed by using a reducing mill to obtain productpipes differing in outer diameter in a range of φ33.2 to 15.0 mm whilesetting the output speed of drawing to 200 m/min. After rolling, thetensile strength (TS) and elongation (E1) were measured on each of theproduct pipes, and the relation between elongation and strength wasshown graphically as is shown in FIG. 1 (plotted by solid circles in thefigure). In the figure, the open circles show the relation betweenelongation and strength of seam welded steel pipes of differing sizewhich were obtained by welding but without applying rolling.

[0015] For the values of elongation (E1), a reduced value obtained bythe following equation:

E1=E10×({square root}{square root over ( )}(a0/a))^(0.4)

[0016] (where, E10 represents the observed elongation, a0 is a valueequivalent to 292 mm², and a represents the cross section area of thespecimen (mm²)).

[0017] Referring to FIG. 1, it can be seen that higher elongation can beobtained if the base steel pipe is subjected to reducing in thetemperature range of from 750 to 550° C. as compared with the elongationof an as-welded seam welded steel pipe at the same strength. That is,the present inventors have been found that a high strength steel pipehaving good balance in ductility and strength can be obtained by heatinga base steel pipe having a specified composition to a temperature rangeof 750 to 400° C. and applying reducing.

[0018] Furthermore, it has been found that the steel pipe produced bythe production method above contain fine ferrite grains 3 μm or less insize. To investigate the collision impact resistance properties thepresent inventors further obtained the relation between the tensilestrength (TS) and the grain size of ferrite while greatly changing thestrain rate to 2,000 s⁻¹. As a result, it has been found that thetensile strength considerably increases with decreasing the ferritegrain diameter to 3 μm or less, and that the increase in TS isparticularly large at the collision impact deformation in case thestrain rate is high. Thus, it has been found additionally that the steelpipe having fine ferrite grains exhibits not only superior balance inductility and strength, but also considerably improved collision impactresistance properties.

[0019] The present invention, which enables a super fine granular steelpipe further reduced in grain size to 1 μm or less, provides a methodfor producing steel comprising heating or soaking a base steel pipehaving an outer diameter of ODi (mm) and having ferrite grains with anaverage crystal diameter of di (μm) in the cross section perpendicularto the longitudinal direction of the steel pipe, and then applyingdrawing at an average rolling temperature of θm (° C.) and a totalreduction ratio Tred (%) to obtain a product pipe having an outerdiameter of ODf (mm),

[0020] wherein, said drawing comprises performing it in the temperaturerange of 400° C. or more but not more than the heating or soakingtemperature, and in such a manner that said average crystal diameter ofdi (μm), said average rolling temperature of θm (° C.), and said totalreduction ratio Tred (%) are in a relation satisfying equation (1) asfollows:

di≦(2.65−0.003×θm)×10^(((0.008+θm/50000)×Tred))  (1)

[0021] where, di represents the average crystal diameter of the basesteel pipe (μm); θm represents the average rolling temperature (° C.)(=(θi+θf)/2; where θi is the temperature of starting rolling (° C.), andθf is the temperature of finishing rolling (° C.)); and Tred representsthe total reduction ratio (%) (=ODi−ODf)×100/ODi; where, ODi is theouter diameter of the base steel pipe (mm), and ODf is the outerdiameter of the product pipe (mm)) In the present invention, thereducing is preferably performed in the temperature range of from 400 to750° C. It is also preferred that the heating or soaking of the basesteel pipe is performed at a temperature not higher than the Ac₃transformation temperature. It is further preferred that the heating orsoaking of the base steel pipe is performed at a temperature in a rangedefined by (Ac₁+50° C.) by taking the Ac₁ transformation temperature asthe reference temperature. Furthermore, the drawing is preferablyperformed under lubrication.

[0022] Preferably, the reducing process is set as such that it comprisesat least one pass having a reduction ratio per pass of 6%, and that thecumulative reduction ratio is 60% or more.

[0023] Furthermore, the method for producing super fine granular steelpipe containing super fine grains having an average grain size of 1 μmor less-according to the present invention preferably utilizes a steelpipe containing 0.70 wt % or less of C as the base steel pipe, and itpreferably a steel pipe containing by weight, 0.005 to 0.30% C., 0.01 to3.0% Si, 0.01 to 2.0% Mn, 0.001 to 0.10% Al, and balance Fe withunavoidable impurities. In the present invention, furthermore, thecomposition above may further contain at least one type selected fromone or more groups selected from the groups A to C shown below:

[0024] Group A: 1% or less of Cu, 2% or less of Ni, 2% or less of Cr,and 1% or less of Mo;

[0025] Group B: 0.1% or less of Nb, 0.5% or less of V, 0.2% or less ofTi, and 0.005% or less of B; and

[0026] Group C: 0.02% or less of REM and 0.01% or less of Ca.

[0027] Additionally, the present inventors have found that, byrestricting the composition of the base steel pipe in a proper range, asteel pipe having high strength and toughness and yet having superiorresistance against stress corrosion cracks can be produced by employingthe above method for producing steel pipes, and that such steel pipescan be employed advantageously as steel pipes for line pipes.

[0028] In order to improve the stress corrosion crack resistanceproperties, conventionally, steel pipes for use in line pipes have beensubjected to hardness control comprising reducing the content ofimpurities such as S or controlling the alloy elements. However, suchmethods had limits in improving the strength, and had problems ofincreasing the cost.

[0029] By further restricting the composition of the base steel pipe toa proper range, and by applying reducing to the base steel pipe in theferritic recrystallization region, fine ferrite grains and fine carbidescan be dispersed as to realize a steel pipe with high strength and hightoughness. At the same time, the alloy elements can be controlled assuch to decrease the weld hardening, while suppressing the generationand development of cracks as to improve the stress corrosion crackresistance.

[0030] That is, the present invention provides a steel pipe havingexcellent ductility and collision impact resistance, yet improved instress corrosion crack resistance by applying drawing under conditionssatisfying equation (1) to abase steel pipe containing, by weight, 0.005to 0.10% C, 0.01 to 0.5% Si, 0.01 to 1.8% Mn, 0.001 to 0.10% Al, andfurther containing at least, one or more types selected from the groupconsisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less ofCr, and 0.5% or less of Mo; or furthermore one or more selected from thegroup consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or lessof Ti, and 0.004% or less of B; or further additionally, one or moreselected from the group consisting of 0.02% or less of REM and 0.01% orless of Ca; and balance Fe with unavoidable impurities.

[0031] Furthermore, the present inventors have found that, byrestricting the composition of the base steel pipe in a further properrange, a steel pipe having high strength and toughness, and yet havingsuperior fatigue resistant properties can be produced by employing theabove method for producing steel pipes, and that such steel pipes can beemployed advantageously as high fatigue strength steel pipes.

[0032] By restricting the composition of the base steel pipe to a properrange, and by applying drawing to the base steel pipe in the ferriticrecovery and recrystallization region, fine ferrite grains and fineprecipitates can be dispersed as to realize a steel pipe with highstrength and high toughness. At the same time, the alloy elements can becontrolled as such to decrease the weld hardening, while suppressing thegeneration and development of fatigue cracks as to improve the fatigueresistance properties.

[0033] That is, the present invention provides a steel pipe havingexcellent ductility and collision impact resistance, yet improved infatigue resistant properties by applying drawing under conditionssatisfying equation (1) to abase steel pipe containing, by weight,0.06,to 0.30% C, 0.01 to 1.5% Si, 0.01 to 2.0% Mn, 0.001 to 0.10% Al,and balance Fe with unavoidable impurities.

[0034] Additionally, it is possible to obtain a high strength steel pipehaving excellent workability, characterized in that it has a compositioncontaining, by weight, more than 0.30% to 0.70% C, 0.01 to 2.0% Si, 0.01to 2.0% Mn, 0.001 to 0.10% Al, and balance Fe with unavoidableimpurities, and a texture consisting of ferrite and a second phase otherthan ferrite accounting for more than 30% in area ratio, with the crosssection perpendicular to the longitudinal direction of the steel pipecontaining super fine grains of said ferrite having an average crystalgrain size of 2 μm or less; otherwise, with the cross sectionperpendicular to the longitudinal direction of the steel pipe containingsuper fine grains of said ferrite having an average crystal grain sizeof 1 μm or less.

BRIEF DESCRIPTION OF THE-DRAWINGS

[0035]FIG. 1 is a graph showing the relation between elongation andtensile strength of the steel pipe;

[0036]FIG. 2 is a graph showing the influence of tensile strain rate onthe relation between the tensile strength and the grain size of ferritecrystals of the steel pipe;

[0037]FIG. 3 is the electron micrograph showing the metallic texture ofthe steel pipe obtained as an example according to the presentinvention;

[0038]FIG. 4 is a schematically drawn diagram of an example of equipmentline according to a preferred embodiment of the present invention;

[0039]FIG. 5 is a schematically drawn diagram of an example of aproduction equipment for solid state pressure welded steel pipes and aproduction line for continuous production according to a preferredembodiment of the present invention;

[0040]FIG. 6 is a graph showing the relation between the total reductionratio and the average crystal grain size of the base steel pipe, whichare the parameters that affect the size reduction of crystal grains ofthe product pipe; and

[0041]FIG. 7 is a schematically drawn explanatory diagram showing theshape of the test specimen for use in sulfide stress corrosion crackresistance test.

[0042] (Explanation of Symbols) 1 Flat strip 2 Pre-heating furnace 3Forming and working apparatus 4 Induction heating apparatus forpre-heating edges 5 Induction heating apparatus for heating edges 6Squeeze roll 7 Open pipe 8 Base steel pipe 14 Uncoiler 15 Joiningapparatus 16 Product pipe 17 Looper 18 Cutter 19 Pipe straighteningapparatus 20 Thermometer 21 Reducing mill 22 Soaking furnace (seamcooling and pipe heating apparatus) 23 Descaling apparatus 24 Quenchingapparatus 25 Re-heating apparatus 26 Cooling apparatus

BEST MODE FOR CARRYING OUT THE INVENTION

[0043] In the present invention, a steel pipe is used as the startingmaterial. There is no particular limitation concerning the method forproducing the base steel pipe. Thus, favorably employable is an electricresistance welded steel pipe (seam welded steel pipe) using electricresistance welding, a solid state pressure welded steel pipe obtained byheating the both edge portions of an open pipe to a temperature regionof solid state pressure welding and effecting pressure welding, a forgewelded steel pipe, or a seamless steel pipe obtained by using Mannesmannpiercer.

[0044] The chemical composition of the base steel pipe or product steelpipe is limited in accordance with the following reasons.

[0045] C: 0.07% or Less:

[0046] Carbon is an element to increase the strength of steel by formingsolid solution with the matrix or by precipitating as a carbide in thematrix. It also precipitates as a hard second phase in the form of finecementite, martensite, or bainite, and contributes in increasingductility (uniform elongation). To achieve a desired strength and toobtain the effect of improved ductility by utilizing cementite and thelike precipitated as the second phase, C must be present at aconcentration of 0.005% or more, and preferably, 0.04% or more.Preferably, the concentration of C is in a range not more than 0.30%,and more preferably, 0.10% or less. In view of these requirements, theconcentration of C is preferably confined in a range of from 0.005 to0.30%, and more preferably, in a range of from 0.04 to 0.30%.

[0047] To improve the stress corrosion crack resistance of the steelpipe to make it suitable for use in line pipes, the concentration of Cis preferably controlled to a range of 0.10% or less. If theconcentration exceeds 0.10%, the stress corrosion crack resistancedecreases due to the hardening of the welded portion.

[0048] To improve the fatigue resistance properties of the steel pipe tomake it suitable for use as a high fatigue strength steel pipe, theconcentration of C is preferably controlled to a range of from 0.06 to30%. If the concentration is lower than 0.06%, the fatigue resistanceproperties decrease due to insufficiently low strength.

[0049] To achieve a desired strength of 600 MPa or more, theconcentration of C must exceed 0.30%. However, if C should beincorporated at a concentration exceeding 0.70%, the ductility isinversely impaired. Thus, the concentration of C should be in a rangeexceeding 0.30% but not more than 0.70%.

[0050] Si: 0.01 to 3.0%:

[0051] Silicon functions as a deoxidizing element, and it increases thestrength of the steel by forming solid solution with the matrix. Thiseffect is observed in case Si is added at a concentration of at 0.01% ormore, preferably at 0.1% or more, but an addition in excess of 3.0%impairs ductility. In case of high strength steel pipe, the upper limitin concentration is set at 2.0% by taking the problem of ductility intoconsideration. Thus, the concentration of Si is set in a range of from0.01 to 3.0%, or of from 0.01 to 2.0%. Preferably, however, the range isfrom 0.1 to 1.5%.

[0052] To improve the stress corrosion crack resistance of the steelpipe to make it suitable for use in line pipes, the concentration of Siis preferably controlled to 0.5% or less. If the concentration exceeds0.5%, the stress corrosion crack resistance decreases due to thehardening of the welded portion.

[0053] To improve the fatigue resistance properties of the steel pipe tomake it suitable for use as a high fatigue strength steel pipe, theconcentration of Si is preferably controlled to 1.5% or less. If theconcentration exceeds 1.5%, the fatigue resistance properties decreasedue to the formation of inclusions.

[0054] Mn: 0.01 to 2.0%:

[0055] Manganese increases the strength of steel, and accelerates theprecipitation of a second phase in the form of fine cementite, ormartensite and bainite. If the concentration is less than 0.01%, notonly it becomes impossible to achieve the desired strength, but alsofine precipitation of cementite or the precipitation of martensite andbainite is impaired. If the addition should exceed 2.0%, the strength ofthe steel is excessively increased to inversely impair ductility. Thus,the concentration of Mn is limited in a range of from 0.01 to 2.0%. Fromthe viewpoint of realizing balance strength and elongation, theconcentration of Mn is preferably is in a range of from 0.2 to 1.3%, andmore preferably, in a range of from 0.6 to 1.3%.

[0056] To improve the stress corrosion crack resistance of the steelpipe to make it suitable for use in line pipes, the concentration of Mnis preferably controlled to 1.8% or less. If the concentration exceeds1.8%, the stress corrosion crack resistance decreases due to thehardening of the welded portion.

[0057] Al: 0.001 t. 0.10%:

[0058] Aluminum provides fine crystal grains. To obtain such finecrystal grains, Al should be added at a concentration of at least0.001%. However, an addition in excess of 0.10% increasesoxygen-containing inclusions which impair the clarity. Thus, theconcentration of Al is set in a range of from 0.001 to 0.10%, andpreferably, in a range of from 0.015 to 0.06%. In addition to the basicsteel composition above, at least one type of an alloy element selectedfrom one or more groups of A to C below may be added.

[0059] Group A: Cu: 1% or Less, Ni: 2% or Less, Cr: 2% or Less, and Mo:1% or Less:

[0060] Any element selected from the group of Cu, Ni, Cr, and Moimproves the quenching property of the steel, and increase the strength.Thus, one or two or more elements can be added depending on therequirements. These elements lowers the transformation point, andeffectively generate fine grains of ferrite or of second phase. However,the upper limit for the concentration of Cu is set at 1%, because Cuincorporated in a large quantity impairs the hot workability. Niincreases not only the strength, but also, toughness. However, theeffect of Ni saturates at an addition in excess of 2%, and an additionin excess increases the cost. Hence, the upper concentration limit isset at 2%. The addition of Cr or Mo in large quantities not only impairsthe weldability, but also increases the total expense. Thus, their upperlimits are set to 2% and 1%, respectively.

[0061] Preferably, the concentration range for the elements in Group Ais from 0.1 to 0.6% for Cu, from 0.1 to 1.0% for Ni, from 0.1 to 1.5%for Cr, and from 0.05 to 0.5% for Mo.

[0062] To make the steel pipes useful for line pipes by improving theresistance againststress corrosioncracks, the concentration of Cu, Ni,Cr, and Mo is each restricted to be 0.5% or lower. If any of them isadded in large quantities as to exceed the concentration of 0.5%,hardening occurs on the welded portion as to degrade the stresscorrosion crack resistance.

[0063] Group B: Nb: 0.1% or Less, V: 0.5% or Less, Ti: 0.2% or Less, andB: 0.005% or Less:

[0064] Any element of the group consisting of Nb, V, Ti, and Bprecipitates as a carbide, a nitride, or a carbonitride, and contributesto the production of fine crystal grains and to a higher strength. Inparticular, for steel pipes which have joints and which are heated tohigh temperatures, these elements function effectively in producing finecrystal grains during heating for joining, or as precipitation nucleifor ferrite during cooling. They are therefore effective in preventinghardening at joint portions. Thus, one or two or more elements can beadded depending on the requirements. However, since their addition inlarge quantities leads to the degradation in weldability and toughness,the upper limits for the concentration of the elements are set asfollows: 0.1% for Nb; 0.5%, preferably 0.3% for V; 0.2% for. Ti; and0.005%, preferably 0.004% for B. More preferably, the concentrationrange for the elements in Group B is from 0.005 to 0.05% for Nb, 0.05 to0.1% for V, from 0.005 to 0.10% for Ti, and from 0.0005 to 0.002% for B.

[0065] To make the steel pipes useful for line pipes by improving theresistance against stress corrosion cracks, the concentration of Nb, V,and Ti is each restricted to be 0.1% or lower. If any of them should beadded in large quantities as to exceed the concentration of 0.1%,hardening occurs on the welded portion as to degrade the stresscorrosion crack resistance.

[0066] Group C: REM: 0.02% or Less, and Ca: 0.01% or Less:

[0067] REM and calcium Ca control the shape of inclusions and improvethe workability. Any element of this group precipitates as a sulfide, anoxide, or a sulfate, and prevents hardening from occurring on the jointportions of steel pipes. Thus, one or more elements can be addeddepending on the requirements. However, if the addition should exceedthe limits of 0.02% for REM and 0.01% for Ca, too many inclusions formas to lower clarity, and degradation in ductility occurs as a result. Itshould be noted that an addition of less than 0.004% for REM, or anaddition of less than 0.001% of Ca exhibits small effect. Hence, it ispreferred that REM are added as such to give a concentration of 0.004%or more, and that Ca is added to 0.001% or more.

[0068] The base steel pipes and product steel pipes contain, in additionto the components described above, balance Fe with unavoidableimpurities. Allowable as the unavoidable impurities are 0.010% or lessof N, 0.006% or less of O, 0.025% or less of P, and 0.020% or less of S.

[0069] N: 0.010% or Less:

[0070] Ni is allowed to a concentration of 0.010%; a quantity necessaryto be combined with Al to produce fine crystal grains. However, anincorporation thereof in excess of this limit impairs the ductility.Hence, it is preferred that the concentration of N is lowered to 0.010%or lower, and more preferably, the concentration thereof is controlledto be in a range of from 0.002 to 0.006%.

[0071] O: 0.006% or Less:

[0072] O impairs clarity by forming oxides. Their incorporation is notdesirable, and its allowable limit is 0.006%.

[0073] P: 0.025% or Less:

[0074] P is preferably not incorporated, because it impairs thetoughness by segregation in grain boundaries. The allowable limitthereof is 0.025%.

[0075] S: 0.020% or Less:

[0076] S is preferably not incorporated, because it increases sulfidesand leads to the degradation of clarity. The allowable limit thereof is0.020%.

[0077] Description on the structure of the product pipes is given below.

[0078] 1) The steel pipe according to the present invention hasexcellent ductility and collision impact resistance properties, andcomprises a texture based on ferrite grains having an average crystaldiameter of 3 μm or less.

[0079] If the size of the ferrite grains exceeds 3 μm, no apparentimprovement can be obtained in ductility as well as in collision impactresistance properties, i.e., the resistance properties against impactweight. Preferably, the average crystal size of ferrite grains is 1 μmor less.

[0080] The average crystal diameter of the ferrite grains in the presentinvention is obtained by observation under an optical microscope or anelectron microscope. More specifically, a cross section obtained bycutting the steel pipe perpendicular to the longitudinal directionthereof, and the observation was made on the etched surface using Nitaletchant. Thus, the diameter of the equivalent circle was obtained for200 or more grains, and the average thereof was used as therepresentative value.

[0081] The structure based on ferrite grains as referred in the presentinvention includes a structure containing solely ferrite and having noprecipitation of a second phase, and a structure containing ferrite anda second phase other than ferrite.

[0082] Mentioned as the second phase other than ferrite are martensite,bainite, and cementite, which may precipitate alone or as a composite oftwo or more thereof. The area ratio of the second phase should accountfor 30% or less. The second phase thus precipitated contributes to theincrease in uniform elongation in case of deformation. Thus, it improvesthe ductility and the collision impact resistance properties. However,such an effect becomes less apparent if the area ratio of the secondphase exceeds 30%.

[0083] 2) The high strength steel pipe according to the presentinvention comprises a structure based on ferrite and a second phaseaccounting for more than 30% in area ratio, and contains grains havingan average crystal diameter of 2 μm or less as observed on a crosssection cut perpendicular to the longitudinal direction of the steelpipe. As the second phase other than ferrite, mentioned are martensite,bainite, and cementite, which may precipitate alone or as a composite oftwo or more thereof The area ratio of the second phase should accountfor more than 30%. The second phase thus precipitated contributes to theincrease in strength and in uniform elongation as to improve thestrength and ductility. However, such an effect is small if the arearatio of the second phase is 30% or less. The area ratio of the secondphase other than ferrite is therefore preferred to be more than 30% butnot more than 60%. If the area ratio should exceed 60%, the ductility isimpaired due to the coarsening of cementite grains.

[0084] If the average crystal diameter should exceed 2 μm, distinctimprovement in ductility is no longer observed, and hence, there is noapparent improvement in the workability. Preferably, the average graindiameter of ferrite is 1 μm or less.

[0085] The average crystal grain diameter according to the presentinvention was obtained by observation under an optical microscope or anelectron microscope. More specifically, a cross section obtained bycutting the steel pipe perpendicular to the longitudinal directionthereof, and the observation was made on the etched surface using Nitaletchant. Thus, the diameter of the equivalent circle was obtained for200 or more grains, and the average thereof was used as therepresentative value. The grain diameter of the second phase is obtainedby taking the boundary of pearlite colony as the grain boundary in casepearlite is the second phase, and, by taking the packet boundary as thegrain boundary in case bainite or martensite is the second phase.

[0086] An example of the steel pipe according to the present inventionis given in FIG. 3.

[0087] The method of producing the steel pipe according to the presentinvention is described below.

[0088] The base steel pipe of the composition described above is *heated in a temperature range of Ac₃ to 400° C., preferably, to a rangeof (Ac₁+50° C.) to 400° C., and more preferably, to a range of 750 to400° C.

[0089] If the heating temperature exceeds the Ac₃ transformation point,not only degradation of the surface properties, but also the coarseningof crystal grains occurs. Accordingly, the heating temperature for thebase steel pipe is preferably set at a temperature not higher than theAc₃ transformation point, preferably, not higher than the (Ac₁+50° C.),and more preferably, not higher than 750° C. On the other hand, if theheating temperature is lower than 400° C., a favorable rollingtemperature cannot be realized. Thus, the heating temperature ispreferably not lower than 400° C.

[0090] Then, the heated base steel pipe is subjected to drawing.

[0091] Although not limiting, drawing is preferably performed by using athree-roll type reducing mill. The reducing mill preferably comprises aplurality of stands, such that rolling is performed continuously. Thenumber of stands can be determined depending on the size of the basesteel pipe and the product steel pipe.

[0092] The rolling temperature for reducing is in a range correspondingto the ferrite recovery and recrystallization temperature range, i.e.,from Ac₃ to 400° C., but preferably, in a range of (Ac₁+50° C.) to 400°C., and more preferably, in a range of from 750 to 400° C. If therolling temperature should exceed the Ac₃ transformation point, no superfine crystal grains would become available, and ductility does notincrease as expected in the expense of decreasing strength. Thus, theroiling temperature is set at a temperature not higher than Ac₃transformation point, preferably, at a temperature not higher than(Ac₁+50° C.), and more preferably, not higher than 750° C. If therolling temperature should be lower than 400° C., on the other hand, thematerial becomes brittle due to blue shortness (brittleness), and mayundergo breakage.

[0093] Furthermore, at rolling temperatures lower than 400° C. not onlythe deformation resistance of the material increases as to make therolling difficult, but also the working strain tends to remain due toinsufficient recovery and recrystallization of the material. Thus, thedrawing is performed in a limited temperature range of from Ac₃ to 400°C., preferably, in a range of (Ac₁+50° C.) to 400° C., and morepreferably, in a range of from 750 to 400° C. Most preferably, thetemperature range is from 600 to 700° C.

[0094] The cumulative reduction ratio in diameter during drawing is setat 20% or higher.

[0095] If the cumulative reduction ratio in diameter, which isequivalent to {[(outer diameter of the base steel pipe)−(outer diameterof the product pipe)]/(outer diameter of the base steel pipe)×100},should be lower than 20%, the crystal grains subjected to recovery andrecrystallization tend to be insufficiently reduced in size. Such asteel pipe cannot exhibit superior ductility. Furthermore, theproduction efficiency becomes low due to the low rate of pipeproduction. Accordingly, in the present invention, the cumulativereduction ratio in diameter is set at 20% or higher. However, at acumulative reduction ratio of 60% or higher, not only an increase instrength due to work hardening occurs, but also fine structure becomesprominent. Thus, even in a steel pipe having a component systemcontaining the alloy elements at a lower concentration than theaforementioned composition range, well balanced strength and ductilitycan be imparted thereto. It can be understood therefrom that, morepreferably, the cumulative reduction ratio in diameter is set at 60% orhigher.

[0096] In performing drawing, it is preferred that the rolling comprisesat least one pass having a diameter reduction ratio per pass of 6% orhigher.

[0097] If the diameter reduction ratio per pass during drawing should beset lower than 6%, fine crystal grains which result from recovery andrecrystallization processes tend to be insufficiently reduced in size.On the other hand, with a diameter reduction ratio per pass of 6% orhigher, an elevation in temperature occurs by the heat of working, whichprevents the drop in temperature from occurring. Thus, the diameterreduction ratio per pass is preferably set at 8% or higher, so that higheffect is obtained in realizing finer crystal grains.

[0098] The drawing process of the steel pipe according to the presentinvention realizes a rolling under biaxial strain, which is particularlyeffective in obtaining fine crystal grains. In contrast to this, therolling of a steel sheet is under uniaxial strain because free end ispresent in the direction of sheet width (i.e., in the directionperpendicular to the rolling direction) Thus, the reduction in grainsize becomes limited.

[0099] In the present invention, it is preferred that drawing isperformed under lubricating conditions, By performing the drawing underlubrication, the strain distribution in the thickness direction becomesuniform that the distribution of crystal size distribution also becomesuniform in the thickness direction. If non-lubricating rolling should beperformed, strain concentrates only on the surface layer portion of thematerial as to disturb the uniformity of the crystal grains in thethickness direction. The lubricating rolling can be carried out by usinga rolling oil well known in the art, for instance, a mineral oil or amineral oil mixed with a synthetic ester can be used without anylimitations.

[0100] After reducing, the steel material is cooled to room temperature.Cooling can be performed by using air cooling, but from the viewpoint ofsuppressing the grain growth as much as possible, any of the coolingmethods known in the art, for instance, water cooling, mist cooling, orforced air cooling, is applicable. The cooling rate is 1° C./sec ormore, and preferably, 10° C./sec. or more. Furthermore, stepwise coolingsuch as holding in the midway of cooling, can be employed depending onthe requirements on the properties of the product.

[0101] In the method according to the present invention, drawing asdescribed below can be applied to the base steel pipe by stablymaintaining the crystal grain diameter of the product pipe to 1 μm orless, or to 2 μm or less in case of a high strength steel pipe.

[0102] Let the average crystal grain diameter of the ferrite grains, or,of that inclusive of the second phase in case of a high strength steelpipe, be di (μm), as observed in the cross section cut perpendicular tothe longitudinal direction of the steel pipe at an outer diameter of ODi(mm). The base steel pipe is then heated or soaked, and is subjected todrawing at an average rolling temperature of θm (° C.) and at a totalreduction ratio in diameter of Tred (%) as to obtain a finished productpipe having an outer diameter of ODf (mm).

[0103] The reducing is preferably applied by using a plurality of passrollers called a reducer. An example of an equipment line suitable forcarrying out the present invention is shown in FIG. 4. In FIG. 4 isshown a rolling apparatus 21 comprising a plurality of stands having apass. The number of stands of the rolling mill is determined properlydepending on the combination in the diameter of the base steel pipe andthe product pipe. For the pass rolls, any type selected from the rollswell known in the art, for instance, two rolls, three rolls, or fourrolls, can be favorably applied.

[0104] There is no particular limitation concerning the heating orsoaking method, however, it is preferred that heating using a heatingfurnace or induction heating is employed. In particular, inductionheating method is preferred from the viewpoint of high heating rate andof high productivity, or from the viewpoint of its ability ofsuppressing the growth of crystal grains. (In FIG. 4 is shown are-heating apparatus 25 of an induction heating type.) The heating orsoaking is performed at a temperature not higher than the Ac₃transformation point corresponding to a temperature range at which nocoarsening of crystal grain occurs, or, at a temperature not higher than(Ac₁+50° C.), by taking the Ac₁ transformation point of the base steelpipe as the standard, or more preferably, in the temperature range offrom 600 to 700° C. In the present invention, as a matter of course, theproduct pipe results with fine crystal grains even if the heating orsoaking of the base steel pipe should be performed at a temperaturedeviating from the temperature range above.

[0105] In case the second phase in the texture of the base steel pipe ispearlite, layered cementite incorporated in pearlite undergoes sizereduction by separation by performing rolling in the temperature rangeabove. Thus, the workability of the product pipe is improved becausebetter elongation properties are acquired. Similarly, in case the secondphase in the structure of the base steel pipe is bainite, the bainiteundergoes recrystallization after working as to form a fine bainiticferrite structure. Thus, the workability of the product pipe is improvedbecause of the improved elongation properties.

[0106] The reducing is performed at a temperature range of 400° C. ormore but not more than the heating or soaking temperature. Preferably,the temperature is not higher than 750° C. The temperature region overthe Ac₃ transformation point, or over (Ac₁+50° C.), or over 750° C.,corresponds to the ferrite-austenite two-phase region rich in austenite,or a single phase region of austenite. Thus, it is difficult to obtain aferritic texture or a texture based on ferrite by working. Moreover, theeffect of producing fine crystal grains by ferritic working cannot befully exhibited. If drawing should be carried out at a temperaturehigher than 750° C., ferrite grains grow considerably afterrecrystallization as to make it difficult to obtain fine grains. In casedrawing is performed at a temperature lower than 400° C., on the otherhand, difficulties are found in carrying out the drawing because thetemperature range corresponds to the blue brittleness region, orductility and toughness decrease because working stress tends to remaindue to insufficient recrystallization. Thus, drawing temperature is setat a temperature not lower than 400° C. but not higher than the Ac₃transformation point, or at a temperature not higher than (Ac₁+50° C.)and preferably, at a temperature not higher than 750° C. Morepreferably, the temperature range is from 560 to 720° C., and mostpreferably, from 600 to 700° C.

[0107] The reducing is performed in the temperature range describedabove, and under the conditions satisfying equation (1), where di (μm)represents the average ferrite crystal diameter as observed in the crosssection perpendicular to the longitudinal direction of the base steelpipe; θm (° C.) represents the average rolling temperature in thedrawing; and Tred (%) represents the total reduction ratio.

[0108] In case di, θm, and Tred do not satisfy the relation expressed byequation (1), the ferrite crystals of the resulting product pipe cannotbe micro-grained as such to yield an average diameter (diameter asobserved in the cross section perpendicular to the longitudinaldirection of the steel pipe) of 1 μm or less. Similarly, the resultinghigh strength steel pipe cannot yield micro-grains as such having anaverage diameter (diameter as observed in the cross sectionperpendicular to the longitudinal direction of the steel pipe) of 2 μmor less.

[0109] Product steel pipes differing in diameter were produced byrolling a JIS STKM 13A equivalent base steel pipe (having an ODi of 60.3mm and a wall thickness of 3.5 mm) by using a rolling apparatusconsisting of serially connected 22 stands of 4-roll rolling mill, andunder the conditions of an output speed is 200 m/min, an average rollingtemperature of 550 or 700° C. The influence of the total reduction ratioin diameter and the average crystal diameter of the base steel pipe onthe crystal grain diameter of the finished product pipe is shown in FIG.6. The conditions shown by the hatched region satisfy the relationexpressed by equation (1) and the base steel pipes with conditionsfalling in this region are capable of providing product pipes comprisingcrystal grains 1 μm or less in diameter.

[0110] After rolling, a product pipe 16 is preferably cooled to atemperature of 300° C. or lower. The cooling can be performed by aircooling, but with an aim to suppress the grain growth as much aspossible, any of the cooling methods known in the art, for instance,water cooling, mist cooling, or forced air cooling, can be applied byusing a quenching apparatus 24. The cooling rate is 1° C./sec or higher,and preferably, 10° C./sec or higher.

[0111] In the present invention, a cooling apparatus 26 may be installedon the input side of a rolling apparatus 21, or in the midway of therolling apparatus 21 to control the temperature. Furthermore, adescaling apparatus 23 may be provided on the input side of the rollingapparatus 21.

[0112] The base steel pipe for use as the starting material in thepresent invention may be any steel pipe selected from a seamless steelpipe, a seam welded steel pipe, a forge welded steel pipe, a solidpressure welded steel pipe, and the like. Furthermore, the productionline of the super fine granular steel pipe according to the presentinvention may be connected to the production line for the base steelpipe described hereinbefore. An example of connecting the productionline to the production line of the solid pressure welded steel pipe isshown in FIG. 5.

[0113] A flat strip 1 output from an uncoiler 14 is connected to apreceding hoop by using a joining apparatus 15, and after beingpreheated by a pre-heating furnace 2 via a looper 17, it is worked intoan open pipe 7 by using a forming apparatus 3 composed of a plurality offorming rolls. The edge portion of the open pipe 7 thus obtained isheated to a temperature region lower than the fusion point by an edgepreheating induction heating apparatus 4 and an edge heating inductionheating apparatus 5, and is butt welded by using a squeeze roll 6 toobtain a base steel pipe 8.

[0114] Then, as described above, the base steel pipe 8 is heated orsoaked to a predetermined temperature by using a soaking furnace 22,descaled by a descaling apparatus 23, rolled by using a rollingapparatus 21, cut by a cutter, and straightened by a pipe straighteningapparatus 19 to finally provide a product pipe 16. The temperature ofthe steel pipe is measured by using a thermometer 20.

[0115] Similarly in the case of drawing, as described above, rolling ispreferably performed under lubrication.

[0116] Thus, in accordance with the production method described above, asteel pipe consisting of super-fine ferrite grains 1 μm or less inaverage crystal grain size as observed in the cross section cutperpendicular to the longitudinal direction of the steel material can beobtained. Furthermore, the production method above is effective inproducing steel pipes, such as seam welded steel pipes, forge weldedsteel pipes, solid pressure welded steel pipes, etc., having a uniformhardness in the seam portion.

[0117] It is also possible to produce, without performing anintermediate annealing, a high strength steel pipe having a texturecomprising ferrite and a second phase other than ferrite accounting formore than 30% in area ratio, and yet consisting of super-fine ferritegrains 2 μm or less in average crystal grain size as observed in thecross section cut perpendicular to the longitudinal direction of thesteel material.

EXAMPLE 1

[0118] Base steel pipes whose chemical composition is shown in Table 1were each heated to temperatures given in Table 2 by using an inductionheating coil, and, by using three-roll structure rolling mills, theywere rolled under conditions shown in Table 2 to provide product pipes.In Table 2, a solid state pressure welded steel pipe was obtained bypre-heating a 2.6 mm thick hot rolled flat strip to 600° C.,continuously forming the resulting flat strip into an open pipe by usinga plurality of rolls, pre-heating the both edge portions of the openpipe to 1,000° C. by means of induction heating, and further heating theboth edge portions to the non-melting temperature region of 1,450° C. byinduction furnace, at which the both ends were butted by using a squeezeroll, where solid phase pressure welding was carried out. Thus wasobtained a steel pipe 42.7 mm in diameter and 2.6 mm in thickness. Onthe other hand, a seamless steel pipe was produced by heating acontinuously cast billet, followed by producing a pipe by using aMannesmann mandrel type mill.

[0119] Tensile properties, collision impact properties, and structure ofthe product pipes were investigated, and the results are given in Table2. Tensile properties were measured on a JIS No. 11 test piece. Yieldstress was obtained by taking the lower yield point in case the yieldphenomenon is clearly observed, but 0.2% PS was used for the othercases.

[0120] For the value of elongation, a reduced value was obtained inaccordance with the following equation by taking the size effect of thetest piece into consideration:

E1=E10×({square root}{square root over ( )}(a0/a))^(0.4)

[0121] (where, E10 represents the observed elongation, a0 is a valueequivalent to 292 mm², and a represents the cross section area of thespecimen (mm²))

[0122] The collision impact properties were obtained by performing highspeed tensile tests at a strain rate of 2,000 s⁻¹. Then, the absorbedenergy up to a strain of 30% was obtained from the observedstress—strain curve to use as the collision impact absorption energy forevaluation.

[0123] The collision impact property is represented by a deformationenergy of a material at a strain rate of from 1,000 to 2,000 s⁻¹practically corresponding to the collision of an automobile, and issuperior for a higher value.

[0124] From Table 2, it can be understood that the specimens falling inthe scope of the present invention (Nos. 1 to 16 and Nos. 19 to 22)exhibit excellent balance in ductility and strength. Moreover, hightensile strength is observed for these specimens having higher strainrate, and these specimens are also high in collision impact absorptionenergy. On the other hand, the specimens falling out of the scope ofclaims according to the present invent ion, i.e., Comparative ExamplesNo. 17, No. 18, and No. 23, suffer low values for either ductility orstrength. These specimens suffer not only poor balance in strengthductility, but also low collision impact property.

[0125] Comparative Example Nos. 17 and 18 furthermore yield a reductionratio falling outside the range according to the present invention, showcoarsening in ferrite grains, and suffer poor balance instrength—ductility and low collision impact absorption energy.

EXAMPLE 2

[0126] Base steel pipes whose chemical composition is shown in Table 3were each heated to temperatures given in Table 4 by using an inductionheating coil, and, by using three-roll structure rolling mills, theywere rolled under conditions shown in Table 4 to provide product pipes.The base steel pipes were produced in the same procedure as thatdescribed in Example 1.

[0127] Tensile properties, collision impact properties, and structure ofthe product pipes were investigated in the same manner as in theExample, and the results are given in Table 4.

[0128] From Table 4, it can be understood that the specimens falling inthe scope of the present invention (Nos. 2-1 to 2-3, Nos. 2-6 to 2-8,and Nos. 2-10 to Nos. 2-14) exhibit excellent balance in ductility andstrength. Moreover, high tensile strength is observed for thesespecimens with higher strain rate, and these specimens are also high incollision impact absorption energy. On the other hand, the specimensfalling out of the scope according to the present invention, i.e.,Comparative Examples No. 2-4, No. 2-5, and No. 2-9, suffer low valuesfor either ductility or strength. These specimens suffer not only poorbalance in strength—ductility, but also low collision impact property.

[0129] The present invention provides steel pipes having not only anever achieved good balance in ductility and strength, but alsoexcellent collision impact resistance properties. Furthermore, the steelpipes according to the present invention exhibit superior properties insecondary working, for instance, bulging such as hydroforming, and aretherefore suitable for use in bulging.

[0130] Among the steel pipes according to the present invention, thewelded steel pipes (seam welded steel pipes) and the solid phasepressure welded steel pipes subjected to seam cooling yield a hardenedseam portion having a hardness at the same level as that of the motherpipe after rolling, and show further distinguished improvement inbulging.

EXAMPLE 3

[0131] Base steel pipes whose chemical composition is shown in Table 5were each heated to temperatures given in Table 6.by using an inductionheating coil, and, by using three-roll structure rolling mills, theywere rolled under conditions shown in Table 6 to provide product pipes.The base steel pipes 110 mm in diameter and 4.5 mm in thickness wereproduced from hot rolled sheet steel produced by controlled rolling andcontrolled cooling.

[0132] Tensile properties, collision impact properties, the structure ofthe product pipes, and sulfide stress corrosion crack resistance wereinvestigated, and the results are given in Table 6. Similar to Example1, tensile properties were measured on a JIS No. 11 test piece. For theelongation, a reduced value was obtained in accordance with thefollowing equation by taking the size effect of the test piece intoconsideration: E1=E10×({square root}{square root over ( )}(a0/a))^(0.4)(where, E10 represents the observed elongation, a0 is a value equivalentto 292 mm² and a represents the cross section area of the specimen(mm²)).

[0133] Similar to Example 1 again, the collision impact properties wereobtained by performing high speed tensile tests at a strain rate of2,000 s⁻¹. Then, the absorbed energy up to a strain of 30% was obtainedfrom the observed stress—strain curve to use as the collision impactabsorption energy for evaluation.

[0134] The collision impact property is represented by a deformationenergy of a material at a strain rate of from 1,000 to 2,000 s⁻¹practically corresponding to the collision of an automobile, and issuperior for a higher value.

[0135] The sulfide stress corrosion crack resistance was evaluated on aC-ring test specimen shown in FIG. 7. Thus, a tensile stresscorresponding to 120% of the yield strength was applied to the specimenin an NACE bath (containing 0.5% acetic acid and 5% brine water,saturated with H₂S, and at a temperature of 25° C. and a pressure of 1atm) to investigate whether cracks generated or not during a test periodof 200 hr. The C-ring specimens were cut out from the mother body of theproduct tube in the T direction (the circumferential direction). Thetest was performed on 2 pieces each under the same condition.

[0136] From Table 6, it can be understood that the specimens falling inthe scope of the present invention (Nos. 3-1 to 3-3, Nos. 3-5 to 3-8,No. 3-10, and No. 3-12) exhibit excellent balance in ductility andstrength. Moreover, high tensile strength is observed for thesespecimens having higher strain rate, and these specimens are also highin collision impact absorption energy. Furthermore, they have excellentresistance against sulfide stress corrosion cracks, and are thereforesuperior when used in line pipes. On the other hand, the specimensfalling out of the scope according to the present invention, i.e.,Comparative Examples No. 3-4, No. 3-9, and No. 3-11, suffer low valuesfor either ductility or strength. These specimens suffer not only poorbalance in strength—ductility, but also low collision impact property.Furthermore, breakage was found to occur on these specimens in the NACEbath, showing degradation in sulfide stress corrosion crack resistance.

[0137] Comparative Example No. 3-4 yields a reduction ratio fallingoutside the range according to the present invention, shows coarseningin ferrite grains, suffers poor balance in strength—ductility and lowcollision impact absorption energy, and exhibits an impaired sulfidestress corrosion crack resistance.

[0138] Comparative Example No. 3-9 and No. 3-11 are produced at arolling temperature falling out of the range according to the presentinvention. Hence, they show coarsening in ferrite grains, suffer poorbalance in strength—ductility and low collision impact absorptionenergy, and exhibit impaired sulfide stress corrosion crack resistance.

EXAMPLE 4

[0139] Base steel pipes whose chemical composition is shown in Table 7were each heated to temperatures given in Table 8 by using an inductionheating coil, and, by using three-roll structure rolling mills, theywere rolled under conditions shown in Table 8 to provide product pipes.The base steel pipes for use in the present example were produced byfirst forming a hot rolled hoop using a plurality of forming rolls toobtain open pipes. Then, seam welded steel pipes 110 mm in diameter and2.0 mm in thickness were produced by welding the both edges of each ofthe resulting open pipes using induction heating. Otherwise, seamlesspipes 110 mm in diameter and 3.0 mm in thickness were produced byheating the continuously cast billets, and then producing pipestherefrom by using a Mannesmann mandrel type mill.

[0140] Tensile properties, collision impact properties, the structure,and the fatigue resistance properties of the product pipes wereinvestigated, and the results are given in Table 8. Tensile properties,collision impact properties, and the structure were evaluated in thesame manner as in Example 1.

[0141] For the fatigue properties, the product pipes were used as theyare for the test specimens, to which cantilever type oscillation fatiguetest was performed (oscillation speed: 20 Hz). Thus, fatigue strengthwas obtained.

[0142] From Table 8, it can be understood that the specimens falling inthe scope the present invention (No. 4-1, No. 4-3, and Nos. 4-6 to 4-9)exhibit excellent balance in ductility and strength. Moreover, hightensile strength is observed for these specimens with higher strainrate, and these specimens are also high in collision impact absorptionenergy. Furthermore, they yield excellent fatigue resistance propertiessuitable for use as high fatigue strength steel pipes. On the otherhand, the specimens falling out of the scope of claims according to thepresent invention, i.e., Comparative Examples No. 4-2, No. 4-4, and No.4-5, suffer low values for fatigue strength.

[0143] Comparative Example No. 4-2 is produced without applying therolling according to the present invention, Comparative Example No. 4-5of yields a reduction ratio falling out of the claimed range, andComparative Example No. 4-4 is rolled at a temperature range out of theclaimed range. Hence, they show coarsening in ferrite grains, sufferpoor balance in strength—ductility and low collision impact absorptionenergy, and exhibit impaired fatigue resistance properties.

EXAMPLE 5

[0144] A starting steel material A1 whose chemical composition is shownin Table 9 was hot rolled to provide a 4.5 mm thick flat strip. By usingthe production line shown in FIG. 5, the flat strip 1 was preheated to600° C. in a preheating furnace 2, and was continuously formed into anopen pipe by using a forming apparatus 3 composed of a plurality ofgroups of forming rolls. The edge portions of each of the open pipes 7thus obtained were heated to 1,000° C. by an edge preheating inductionheating apparatus 4, and were then heated to 1,450° C. by using an edgeheating induction heating apparatus 5, where they were butted and solidphase pressure welded by using squeeze rolls 6 to obtain base steelpipes 8 having a diameter of 88.0 mm and a thickness of 4.5 mm.

[0145] Then, each of the base steel pipes was subjected to seam cooling,and was heated or soaked to a predetermined temperature shown in Table10 by using a pipe heating apparatus 22, and a product pipe having thepredetermined outer diameter was produced there from by using a rollingapparatus 21 composed of a plurality of three-roll structured rolling,mill. The number of stands was varied depending on the outer diameter ofthe product pipe; i.e., 6 stands were used for a product pipe having anouter diameter of 60.3 mm, whereas 16 stands were used for those havingan outer diameter of 42.7 mm.

[0146] In the rolling step above, the product pipe of No. 5-2 wassubjected to lubrication rolling by using a rolling oil based on mineraloil mixed with a synthetic ester.

[0147] The product pipes were air cooled after rolling.

[0148] Crystal grain diameter, tensile properties, and impact resistanceproperties were investigated for each of the product pipes thusobtained, and the results are given in Table 10. The crystal-graindiameter was obtained by microscopic observation under a magnificationof 5,000 times of at least 5 vision fields taken on a cross section (Ccross section) perpendicular to the longitudinal direction of the steelpipe, thus measuring the average crystal grain diameter of ferritegrains. Tensile properties were measured on a JIS No. 11 test piece. Forthe elongation, a reduced value was obtained in accordance with thefollowing equation by taking the size effect of the test piece intoconsideration: E1=E10×({square root}{square root over ( )}(a0/a))^(0.4)(where, E10 represents the observed elongation, a0 is a value equivalentto 100 mm², and a represents the cross section area of the specimen(mm²)). Impact properties (toughness) were evaluated by subjecting theactual pipe to Charpy impact tests, and by using the ductile ruptureratio in C cross section at a temperature of −150° C. Charpy impact teston an actual pipe was performed by applying impact to an actual pipeV-notched for 2 mm in a direction perpendicular to the longitudinaldirection of the pipe, and the ratio of ductile rupture was obtainedtherefrom.

[0149] From Table 10, it can be understood that the specimens falling inthe scope of the present invention (No. 5-2, Nos. 5-4 to 5-7,. Nos. 5-9to 5-11, and No. 5-13) consist of fine ferrite grains 1 μm or less inaverage crystal diameter, have high elongation and toughness, andexhibit excellent balance in strength, toughness, and ductility. In caseof specimen No. 5-2 subjected to lubrication rolling, small fluctuationwas observed in crystal grains along the direction of pipe thickness. Onthe other hand, the specimens falling out of the scope according to thepresent invention, i.e., the Comparative-Examples (No. 5-1, No. 5-3, No.5-8., and No. 5-12), exhibit coarsened crystal grains and sufferdegradation in ductility and toughness. It has been found that thetexture of the product pipes falling in the scope of claims of thepresent invention consists of ferrite and pearlite grains, ferrite andcementite grains, or ferrite and bainite grains.

EXAMPLE 6

[0150] A steel material B1 whose chemical composition is shown in Table9 was molten in a converter, and billets were formed therefrom bycontinuous casting. The resulting billets were heated, and seamlesspipes 110.0 mm in diameter and 6.0 mm in thickness were obtainedtherefrom by using a Mannesmann mandrel type mill. The seamless pipesthus obtained were re-heated to temperatures shown in Table 11 by usinginduction heating coils, and product pipes having the outer diametershown in Table 11 were produced therefrom by using a three-rollstructured rolling mill. The number of stands was varied depending onthe outer diameter of the product pipe; i.e., 18 stands were used for aproduct pipe having an outer diameter of 60.3 mm, 20 stands were usedfor a product pipe 42.7 mm in diameter, 24 stands were used for aproduct pipe 31.8 mm in diameter, and 28 stands were used for thosehaving an outer diameter of 25.4 mm.

[0151] The characteristic properties of the product pipes were eachinvestigated and are shown in Table 11. Thus, investigations were madein the same manner as in Example 5 on the structure, crystal grain size,tensile properties, and toughness.

[0152] From Table 11, it can be understood that the specimens falling inthe scope of the present invention (No. 6-1, No. 6-3, No. 6-6, No. 6-7,and No. 6-9) consist of fine ferrite grains 1 μm or less in averagecrystal diameter, have high elongation and toughness, and exhibitexcellent balance in strength, toughness, and ductility. On the otherhand, the specimens falling out of the scope according to the presentinvention, i.e., the Comparative Examples (No. 6-2, No. 6-4, No. 6-5,and No. 6-8), exhibit coarsened crystal grains and suffer degradation inductility and toughness.

[0153] It has been found that the texture of the product pipes fallingin the scope of claims of the present invention consists of ferrite andpearlite grains, ferrite and cementite grains, or ferrite and bainitegrains.

EXAMPLE 7

[0154] Starting steel materials whose chemical composition is shown inTable 12 were each heated to temperatures given in Table 13 by using aninduction heating coil, and, by using three-roll structure rollingmills, they were rolled under conditions shown in Table 13 to provideproduct pipes. The number of stands was varied depending on the type ofthe pipe; i.e., 24 stands were used for seamless pipes, whereas 16stands were used for solid phase pressure welded pipes and seam weldedpipes.

[0155] In Table 13, a solid state pressure welded steel pipe wasobtained by pre-heating a 2.3 mm thick hot rolled flat strip to 600° C.,continuously forming the resulting flat strip into an open pipe by usinga plurality of rolls, pre-heating the both edge portions of the openpipe to 1,000° C. by means of induction heating, further heating theboth edge portions by induction furnace to a temperature of 1,450° C.,i.e., to a temperature below the melting, at which the both ends werebutted by using a squeeze roll, and carrying out solid phase pressurewelding. Thus was obtained the steel pipes having the predeterminedouter diameter. On the other hand, seamless steel pipes were produced byheating a continuously cast billet, and producing therefrom the seamlesspipes 110.0 mm in diameter and 4.5 mm in thickness by using a Mannesmannmandrel type mill.

[0156] The characteristic properties of the product pipes were eachinvestigated and are shown in Table 13. Thus, investigations were madein the same manner as in Example 1 on the structure, crystal grain size,tensile properties, and toughness.

[0157] From Table 13, it can be understood that the specimens falling inthe scope of the present invention consist of fine ferrite grains 1 μmor less in average crystal diameter, have high elongation and toughness,and exhibit excellent balance in strength, toughness, and ductility. Ithas been found that the structure of the product pipes falling in thescope of claims of the present invention consists of ferrite andpearlite grains, or of ferrite, pearlite, and bainite grains, or offerrite and cementite grains, or of ferrite and martensite grains.

EXAMPLE 8

[0158] Each of the starting steel materials whose chemical compositionis shown in Table 14 was hot rolled to provide a 4.5 mm thick flatstrip. By using the production line shown in FIG. 5, the flat strip 1was preheated to 600° C. in a preheating furnace 2, and was continuouslyformed into an open pipe by using a forming apparatus 3 composed of aplurality of groups of forming rolls. The edge portions of each of theopen pipes 7 thus obtained were heated to 1,000° C. by an edgepreheating induction heating apparatus 4, and were then heated to 1,450°C. by using an edge heating induction heating apparatus 5, where theywere butted and solid phase pressure welded by using squeeze rolls 6 toobtain base steel pipes 8 having a diameter of 110.0 mm and a thicknessof 4.5 mm.

[0159] Then, each of the base steel pipes was subjected to seam cooling,and was heated or soaked to a predetermined temperature shown in Table15 by using a pipe heating apparatus 22, and a product pipe having thepredetermined outer diameter was produced therefrom by using a rollingapparatus 21 composed of a plurality of three-roll structured rollingmill. The number of stands was varied depending on the outer diameter ofthe product pipe; i.e., 6 stands were used for a product pipe having anouter diameter of 60.3 mm, whereas 16 stands were used for those havingan outer diameter of 42.7 mm.

[0160] In the rolling step above, the product pipe of No. 1-2 wassubjected to lubrication rolling by using a rolling oil based on mineraloil mixed with a synthetic ester.

[0161] The product pipes were air cooled after rolling.

[0162] Crystal grain diameter and tensile properties were investigatedfor each of the product pipes thus obtained, and the results are givenin Table 15. The crystal grain diameter was obtained by microscopicobservation under a magnification of 5,000 times of at least 5 visionfields taken on a cross section (C cross section) perpendicular to thelongitudinal direction of the steel pipe, thus measuring the averagecrystal grain diameter of ferrite grains. Tensile properties weremeasured on a JIS No. 11 test piece. For the elongation, a reduced valuewas obtained in accordance with the following equation by taking thesize effect of the test piece into consideration: E1=E10×({squareroot}{square root over ( )}(a0/a)^(0.4) (where, E10 represents theobserved elongation, a0 is a value equivalent to 100 mm², and arepresents the cross section area of the specimen (mm²)).

[0163] From Table 15, it can be understood that the specimens falling inthe scope of the present invention (No. 1-2, Nos. 1-4 to 1-7, and No.1-10) consist of fine grains 2 μm or less in average crystal diameter,have high elongation and toughness, yield a tensile strength of 600 MPaor higher, and exhibit excellent balance in strength, toughness, andductility.

[0164] In case of specimen No. 1-2 subjected to lubrication rolling,small fluctuation was observed in crystal grains along the direction ofpipe thickness. On the other hand, the specimens falling out of thescope according to the present invention, i.e., the Comparative Examples(No. 1-1, No. 1-3, No. 1-8, and No. 1-9), exhibit coarsened crystalgrains and suffer degradation in ductility.

[0165] It has been found that the texture of the product pipes fallingin the scope of claims of the present invention comprises ferrite, andcementite which accounts for more than 30% in area ratio as a secondphase.

EXAMPLE 9

[0166] Each of the base steel pipes whose chemical composition is shownin Table 16 was re-heated by an induction heating coil to temperaturesshown in Table 17, and product pipes each having the outer diametershown in Table 17 were each obtained therefrom by using a three-rollstructure rolling mill apparatus. The number of stands used in therolling mill was 16.

[0167] The characteristic properties of the product pipes were eachinvestigated and are shown in Table 17. Thus, investigations were madein the same manner as in Example 8 on the texture, crystal grain size,and tensile properties.

[0168] From Table 17, it can be understood that the specimens (Nos. 2-1to 2-6) falling in the scope of the present invention consist of fineferrite grains 2 μm or less in average crystal diameter, yield a tensilestrength of 600 MPa or higher, have high elongation, and exhibitexcellent balance in strength and ductility. On the other hand, thespecimens falling out of the scope according to the present invention,i.e., the Comparative Examples (No. 2-7 and No. 2-8), exhibit coarsenedcrystal grains and suffer degradation in strength that a targetedtensile strength is not obtained.

[0169] It has been found that the texture of the product pipes fallingin the scope of the present invention comprises ferrite, and a secondphase containing pearlite, cementite, bainite, or martensite, whichaccounts for more than 30% in area ratio.

[0170] As described above, the present invention provides high strengthsteel pipes considerably improved in balance of ductility and strength.Moreover, the steel pipes according to the present invention exhibitsuperior properties in secondary working, for instance, bulging such ashydroforming. Hence, they are particularly suitable for use in bulging.

[0171] Among the steel pipes according to the present invention, thewelded steel pipes and the solid state pressure welded steel pipessubjected to seam cooling yield a hardened seam portion having ahardness at the same level as that of the mother pipe after rolling, andshow further distinguished improvement in bulging. TABLE 1 SteelChemical Composition (wt %) Ac₁ Ac₃ No. C Si Mn P S Al N O ° C. ° C.Note A 0.09 0.40 0.80 0.012 0.005 0.035 0.0035 0.0025 770 900 InventionB 0.08 0.07 1.42 0.015 0.011 0.036 0.0038 0.0036 760 875 Invention C0.06 0.21 0.35 0.013 0.008 0.028 0.0025 0.0028 775 905 Invention D 0.110.22 0.45 0.017 0.013 0.018 0.0071 0.0035 775 885 Invention E 0.21 0.200.50 0.016 0.013 0.024 0.0043 0.0030 770 855 Invention F 0.03 0.05 0.150.021 0.007 0.041 0.0026 0.0038 780 905 Invention G 0.09 0.15 0.52 0.0240.003 0.004 0.0025 0.0026 775 890 Invention

[0172] TABLE 2 Conditions of reduction rolling Base steel pipe Temp. ofTemp. of Cumulative Outer Heating starting finishing reduction No. ofFinal rolling Outer diameter Steel diameter temp. rolling rolling ratioTotal No. pass 6% speed of pipe product No. No. Type mm ° C. ° C. ° C. %of pass or more m/min mm 1 A Solid phase pressure 42.7 750 710 690 65 149 200 15.0 welded pipe 2 A Solid phase pressure 42.7 700 670 660 65 14 9200 15.0 welded pipe 3 A Solid phase pressure 42.7 650 635 620 65 14 9200 15.0 welded pipe 4 A Solid phase pressure 42.7 700 655 630 40 7 4140 25.5 welded pipe 5 A Solid phase pressure 42.7 650 605 590 40 7 4140 25.5 welded pipe 6 A Solid phase pressure 42.7 700 660 630 30 5 3120 29.7 welded pipe 7 A Solid phase pressure 42.7 650 615 590 30 5 3120 29.7 welded pipe 8 A Solid phase pressure 42.7 700 660 640 22 3 2110 33.2 welded pipe 9 A Solid phase pressure 42.7 650 615 585 22 3 2110 33.2 welded pipe 10 A Solid phase pressure 42.7 650 620 580 22 7 0110 33.2 welded pipe Characteristics of pipe product Tensile strengthElongation High speed tensile Collision Impact Ferrite grain Area ratioof Type of TS El strength absorped energy diameter second phase secondMPa % MPa MJ-m³ μm % phase* Miscellaneous Note 525 44 728 242 2.0 10 CInvention 575 43 780 260 2.0 11 C Invention 622 40 864 292 1.0 11 CInvention 537 43 761 257 1.0 11 C Invention 580 38 799 267 1.5 11 CInvention 512 40 724 241 1.5 11 C Invention 562 38 799 268 1.0 11 CInvention 493 42 712 230 1.0 11 C Invention 541 39 755 249 1.5 11 CInvention 537 36 751 242 1.5 11 C Invention Conditions of reductionrolling Base steel pipe Temp. of Temp. of Cumulative Outer Heatingstarting finishing reduction No. of Final rolling Outer diameter Steeldiameter temp. rolling rolling ratio Total No. pass 6% speed of pipeproduct No. No. Type mm ° C. ° C. ° C. % of pass or more m/min mm 11 BSeam welded steel 42.7 650 650 622 65 14 9 200 15.0 pipe 12 B Seamwelded steel 42.7 600 590 580 65 14 9 200 15.0 pipe 13 C Seam weldedsteel 42.7 650 640 620 65 14 9 200 15.0 pipe 14 D Seamless steel pipe110 700 695 670 77 17 10 150 25.6 15 E Seamless steel pipe 110 700 695670 77 17 10 150 25.6 16 A Solid phase pressure 42.7 550 540 528 85 14 9200 15.0 welded pipe 17 C Seam welded steel 42.7 — — — 0 — — — 42.7 pipe18 C Seam welded steel 42.7 650 630 615 11 3 1 80 38.0 pipe 19 F Seamwelded steel 42.7 650 600 545 65 14 9 200 15.0 pipe 20 G Seam weldedsteel 42.7 750 705 690 65 14 9 200 15.0 pipe 21 G Seam welded steel 42.7650 620 615 65 14 9 200 15.0 pipe 22 G Seam welded steel 42.7 750 710685 41 7 4 140 25.3 pipe 23 G Seam welded steel 42.7 950 910 890 22 3 2110 33.1 pipe Characteristics of pipe product Tensile strengthElongation High speed tensile Collision Impact Ferrite grain Area ratioof Type of TS El strength absorped energy diameter second phase secondMPa % MPa MJ-m³ μm % phase* Miscellaneous Note 555 42 792 265 1.0 15 CInvention 611 37 850 289 1.0 15 C Invention 492 42 685 225 2.5 7 CInvention 475 52 666 219 2.0 9 C Invention 526 46 733 231 2.0 22 C + 8Invention 688 30 892 299 2.5 12 C Invention 409 43 566 185 11.0 6 P **Comparative 427 40 570 191 7.0 8 C Invention 552 29 744 248 3.0 0 —Invention 431 48 611 202 3.0 13 C Invention 511 33 704 233 3.0 13 CInvention 425 47 604 206 3.0 12 C Invention 410 45 570 183 18.0 13 CComparative

[0173] TABLE 3 Steel Chemical composition (wt. %) No. C Si Mn P S Al N OCu Ni Cr Mo V H 0.07 0.20 0.66 0.018 0.005 0.028 0.0022 0.0025 — — — — —I 0.08 0.04 1.35 0.015 0.011 0.036 0.0041 0.0032 — — — — 0.10 J 0.150.21 0.55 0.009 0.004 0.010 0.0028 0.0028 — — 0.21 0.53 — K 0.05 1.011.35 0.012 0.001 0.035 0.0030 0.0030 — — 0.92 — — L 0.15 0.22 0.41 0.0180.003 0.031 0.0036 0.0038 0.11 0.15 — — — Chemical Steel composition (wt%) Ac₁ Ac₃ No. Nb Ti B Ca ° C. ° C. Note H 0.009 0.008 — — 765 895Invention I — — — 0.002 755 885 Invention J — — — — 785 890 Invention K0.015 0.011 0.0023 — 790 905 Invention L — — — 0.002 760 875 Invention

[0174] TABLE 4 Conditions of reduction rolling Base steel pipe Temp. ofTemp. of Cumulative Outer Heating starting finishing reduction No. ofFinal rolling Outer diameter Steel diameter temp. rolling rolling ratioTotal No. pass 6% speed of pipe product No. No. Type mm ° C. ° C. ° C. %of pass or more m/min mm 2-1 H Solid phase pressure 42.7 730 700 640 6514 9 200 15.0 welded pipe 2-2 Solid phase pressure 42.7 670 640 600 6514 9 200 15.0 welded pipe 2-3 Solid phase pressure 42.7 620 600 560 6514 9 200 15.0 welded pipe 2-4 Solid phase pressure 42.7 — — — 0 — — —42.7 welded pipe 2-5 Solid phase pressure 42.7 670 640 600 11 3 1 8038.0 welded pipe 2-6 I Solid phase pressure 42.7 700 670 620 41 7 4 14025.3 welded pipe 2-7 Solid phase pressure 42.7 800 780 770 41 7 4 14025.3 welded pipe 2-8 Solid phase pressure 42.7 850 830 820 41 7 4 14025.3 welded pipe 2-9 Solid phase pressure 42.7 950 930 910 41 7 4 14025.3 welded pipe 2- J Seamless steel pipe 110 700 700 690 69 17 15 40034.1 10 2- K Seam welded steel 42.7 720 690 650 65 14 9 200 15.0 11 2- LSeamless steel pipe 110 700 700 680 77 24 18 400 25.4 12 2- Seamlesssteel pipe 110 800 780 770 77 24 18 400 25.4 13 2- Seamless steel pipe110 850 830 820 77 24 18 400 25.4 14 Characteristics of pipe productTensile strength Elongation High speed tensile Collision Impact Ferritegrain Area ratio of Type of TS El strength absorped energy diametersecond phase second MPa % MPa MJ-m³ μm % phase* Miscellaneous Note 53043 734 242 2.0 8 C Invention 640 38 884 301 1.0 7 C Invention 730 32 931318 2.0 8 C Invention 470 40 640 196 7.0 7 C ** Comparative 490 37 666199 6.0 6 C Comparative 530 40 724 240 2.5 13 C Invention 500 44 682 2232.5 12 C Invention 480 41 644 205 2.8 14 C + P Invention 390 40 532 1306.5 15 P Comparative 663 42 885 298 1.5 23 C + B Invention 712 34 931318 1.5 12 M Invention 581 44 802 259 1.5 18 C Invention 556 46 757 2362.0 20 C Invention 500 40 658 210 2.5 21 C + P Invention

[0175] TABLE 5 Steel Chemical composition (wt. %) No. C Si Mn P S Al N OCu Ni Cr Mo V M 0.05 0.30 1.22 0.007 0.001 0.022 0.0030 0.0028 — 0.20 —0.05 0.05 N 0.08 0.51 1.41 0.008 0.001 0.028 0.0035 0.0019 0.12 0.180.15 — 0.02 O 0.06 0.28 0.95 0.009 0.001 0.025 0.0026 0.0025 — 0.15 —0.06 0.02 P 0.06 0.30 1.18 0.008 0.001 0.028 0.0031 0.0023 0.15 0.15 — —0.04 Q 0.04 0.10 1.50 0.006 0.001 0.018 0.0029 0.0023 — — — 0.06 0.06Chemical Steel composition (wt. %) Ac₁ Ac₃ No. Nb Ti B Ca REM ° C. ° C.Note M 0.05 0.011 — — — 770 895 Invention N 0.02 0.007 0.011 — — 760 890Invention O 0.03 0.009 — 0.002 — 770 900 Invention P 0.03 0.009 — —0.007 765 900 Invention Q 0.04 — — — — 770 885 Invention

[0176] TABLE 6 Base steel pipe Conditions of reduction rolling OuterHeating Temp. of Temp. of Cumulative Outer diameter Steel diameter temp.starting rolling finishing rolling reduction ratio Total No. No. of passof pipe product No. No. Type mm ° C. ° C. ° C. % of pass 6% or more mm3-1 M Seam welded 110 720 700 680 45 10 7 60.5 3-2 steel pipe 660 650640 45 10 7 60.5 3-3 610 600 590 45 10 7 60.5 3-4 660 650 640 8 3 1101.6 3-5 N 660 650 640 45 10 7 60.5 3-6 O 720 700 690 69 17 15 34.1 3-7800 780 770 69 17 15 34.1 3-8 850 830 820 69 17 15 34.1 3-9 950 920 90069 17 15 34.1 3-10 P 720 690 650 69 17 15 34.1 3-11 950 920 900 69 17 1534.1 3-12 Q 720 700 680 77 24 18 25.4 Characteristics of pipe productTensile High speed Presence of Yield strength Elongation tensileCollision impact SSC Ferrite grain Area ratio of Type of strength*** TSEl strength absorption energy resistant diameter second phase secondMiscel- MPa MPa % MPa MJ-m³ cracks*** μm % phase* laneous Note 507 61641 786 258 ∘ ∘ 2.0 5 C Invention 565 642 38 838 275 ∘ ∘ 1.5 5 CInvention 616 692 35 906 293 ∘ ∘ 2.0 5 C Invention 506 582 43 761 199 ∘x 10.0 5 C Comparative 637 724 35 943 307 ∘ ∘ 2.0 20 C Invention 560 62542 815 270 ∘ ∘ 1.5 5 C Invention 538 611 43 772 250 ∘ ∘ 2.0 5 CInvention 521 593 45 733 230 ∘ ∘ 2.5 5 C Invention 431 538 39 668 180 x∘ 6.0 B C + B Comparative 582 640 40 830 273 ∘ ∘ 1.5 5 C Invention 445550 39 678 180 x x 6.5 7 C + B Comparative 600 658 38 861 279 ∘ ∘ 1.5 5C Invention

[0177] TABLE 7 Steel Chemical composition (wt. %) No. C Si Mn P S Al N OCu Ni Cr Mo V R 0.09 0.02 0.73 0.011 0.003 0.032 0.0036 0.0025 — — — — —S 0.11 0.15 1.28 0.007 0.001 0.028 0.0041 0.0025 0.12 0.18 0.15 — — T0.14 0.35 0.91 0.008 0.001 0.025 0.0038 0.0033 — — — — 0.02 U 0.12 0.251.36 0.008 0.001 0.028 0.0030 010028 — — — — — V 0.21 0.20 0.48 0.0090.001 0.025 0.0038 0.0031 0.12 0.12 0.11 0.05 0.02 Chemical Steelcomposition (wt. %) Ac₁ Ac₃ No. Nb Ti B Ca REM ° C. ° C. Note R — — — —— 770 880 Invention S — — — — — 755 850 Invention T 0.021 0.007 0.0011 —— 770 870 Invention U — — — 0.003 — 760 865 Invention V 0.009 0.009 — —0.006 765 840 Invention

[0178] TABLE 8 Base steel pipe Condition of reduction rolling Temp. ofTemp. of Cumulative Outer Heating starting finishing reduction Outerdiameter Steel diameter temp. rolling rolling ratio Total No. No. ofpass of pipe product No. No. Type mm ° C. ° C. ° C. % of pass 6% or moremm 4-1 R Seam welded 110 660 650 630 68 14 9 35.0 4-2 steel pipe 35.0—** 35.0 4-3 S 110 605 600 590 68 14 9 35.0 4-4 880 860 830 68 14 9 35.04-5 660 650 640 18 4 2 90.0 4-6 700 690 670 77 17 10 25.6 4-7 T Seamlesssteel 110 660 650 630 77 17 10 25.6 4-8 U pipe 660 650 630 77 17 10 25.64-9 V 660 650 630 77 17 10 25.6 Characteristics of pipe product Yieldstrength Tensile Elongation High speed Collision impact Fatigue strengthFerrite grain Area ratio of Type of *** strength El tensile strengthabsorbed energy **** diameter second phase second MPa TS MPa % MPaMJ-m⁻³ MPa μm % phase ** Note 466 550 47 742 198 220 1.5 14 C Invention364 448 45 553 124 140 13.0 15 C Comparative 531 612 40 821 223 250 1.518 C Invention 421 517 38 648 143 155 8.0 16 C + B Comparative 451 52236 679 151 160 9.0 18 C Comparative 525 575 42 761 255 250 0.9 18 CInvention 507 596 40 795 196 235 2.0 16 C Invention 523 618 39 806 198240 2.5 20 C Invention 570 657 37 850 210 255 2.0 23 C Invention

[0179] TABLE 9 Steel Chemical composition (wt. %) No. C Si Mn P S Al NA1 0.06 0.05 0.35 0.018 0.019 0.028 0.0025 B1 0.25 0.20 0.82 0.012 0.0070.010 0.0028

[0180] TABLE 10 Outer dia- Total meter of Crystal grain Base Conditionsof reduction rolling Outer reduc- base diameter of steel pipe HeatingTemp. of Temp. of Av. rolling diameter of tion Equation (1) Steel pipebase pipe Ac₁ Ac₃ temp. starting rolling finishing rolling temp. pipeproduct ratio Left Right No. No. mm μm ° C. ° C. ° C. ° C. ° C. ° C. mm% side side 5-1 88.0 3.8 400 395 412 404 42.7 51.5 3.8 9.67 5-2 A1 450445 458 452 60.3 31.5 3.8 4.45 5.3 770 900 670 660 641 651 60.3 31.5 3.83.20 5-4 670 660 638 649 42.7 51.5 3.8 8.45 5-5 810 775 748 762 42.751.5 3.8 5.74 5-6 8.2 450 445 462 454 42.7 51.5 8.2 9.75 5-7 600 590 592591 42.7 51.5 8.2 9.19 5-8 670 660 639 650 60.3 31.5 8.2 3.21 5-9 670660 636 648 42.7 51.5 8.2 8.47 5-10 735 720 702 711 31.8 63.9 8.2 13.575-11 780 760 737 749 31.8 63.9 8.2 11.85 5-12 13.1 450 445 458 452 42.751.5 13.1 9.75 5-13 445 440 465 453 31.8 63.9 13.1 15.86 Characteristicsof pipe product Crystal grain Yield strength Tensile strength ElongationReal pipe Area ratio of diameter YS TS (EL) Charpy ductile rupture ratiosecond phase μm MPa MPa % % Structure* % Note Breakage occurred duringrolling Comparative 0.92 613 648 41 90 F + P P:8 Invention 2.25 496 53832 40 F + C C:6 Comparative 0.55 431 518 48 100 F + C C:6 Invention 0.99415 448 38 75 F + B B:8 Invention 0.95 552 597 41 90 F + P P:8 Invention0.81 451 502 44 95 F + P P:6 Invention 5.12 451 485 28 0 F + C 0:5Comparative 0.68 439 506 46 100 F + C C:5 Invention 0.78 448 496 44 95F + B B:8 Invention 0.90 413 462 43 90 F + B B:8 Invention 6.92 560 57423 0 F + P P:8 Comparative 0.96 607 656 42 90 F + P P:8 Invention

[0181] TABLE 11 Outer Crystal Total diameter grain Base steel Conditionsof reduction rolling Outer reduc- of base diameter pipe Heating Temp. ofTemp. of Av. rolling diameter of tion Equation (1) Steel pipe of pipeAc₁ Ac₃ temp. starting rolling finishing rolling temp. pipe productratio Left Right No. No. mm pipe μm ° C. ° C. ° C. ° C. ° C. ° C. mm %side side 6-1 110.0 6.3 625 615 591 603 60.3 45.2 6.3 6.78 6-2 B1 735720 690 705 60.3 45.2 6.3 5.33 6-3 765 830 735 720 684 702 42.7 61.2 6.312.14 6-4 15.2 560 550 553 552 42.7 61.2 15.2 14.53 6-5 675 665 640 65342.7 61.2 15.2 3.44 6-6 680 670 637 654 31.8 71.1 15.2 21.70 6-7 785 765726 746 31.8 71.1 15.2 17.59 6-6 28.1 680 670 637 654 31.8 71.1 28.121.70 6-9 680 675 634 655 25.4 76.9 28.1 28.75 Characteristics of pipeproduct Crystal grain Yield point Tensile strength Elongation Real pipeArea ratio of second diameter YS TS (EL) Charpy ductile rupture ratiophase μm MPa MPa % % Structure* % Note 0.82 589 660 42 95 F + P P:23Invention 2.13 486 532 37 20 F + B B:25 Comparative 0.91 513 588 43 90F + B B:20 Invention 2.36 601 643 41 20 F + P P:23 Comparative 3.22 564602 34 10 F + C C:16 Comparative 0.57 592 671 44 100 F + C C:16Invention 0.88 568 623 46 90 F + B B:23 Invention 4.96 596 642 24 0 F +C C:18 Comparative 0.69 638 711 42 100 F + C C:18 Invention

[0182] TABLE 12 Steel Chemical composition (wt. %) No. C Si Mn P S Al NCu Ni Cr Mo V Nb Ti B Ca REM C1 0.09 0.40 0.80 0.012 0.005 0.035 0.0035— — — — — — — — — — D1 0.21 0.20 0.50 0.016 0.013 0.024 0.0043 — — — — —— — — — — E1 0.15 0.21 0.55 0.009 0.004 0.010 0.0028 — — 0.21 0.53 — — —— — — F1 0.15 0.22 0.45 0.018 0.003 0.031 0.0036 0.11 0.15 — — — — — —0.002 — G1 0.08 0.04 1.35 0.015 0.011 0.036 0.0041 — — — — 0.10 — — —0.002 — H1 0.05 1.01 1.35 0.012 0.001 0.035 0.0030 — — — — — 0.015 0.0110.0023 — — I1 0.14 0.30 1.30 0.011 0.003 0.028 0.0038 0.20 0.25 — — — —— — — 0.008

[0183] TABLE 13 Base steel pipe Outer Total Outer Crystal Conditions ofreduction rolling diameter reduc- dia- grain Heating Temp. of Temp. ofAv. rolling of pipe tion Equation (1) Steel meter diameter Ac₁ Ac₃ temp.starting finishing temp. product ratio Left Right No. No. Type mm μm °C. ° C. ° C. ° C. rolling ° C. rolling ° C. mm % side side 7-1 C1 Solidphase 88.0 6.3 770 895 450 443 460 452 60.3 31.5 3.8 4.45 7-2 pressure8.2 600 589 593 591 42.7 51.5 8.2 9.19 7-3 D1 welded 13.1 760 850 445437 469 453 31.8 63.9 13.1 15.66 7-4 pipe 13.1 690 670 620 650 42.7 51.46.3 6.81 7-5 E1 Seamless 110.0 6.3 765 880 625 610 596 603 60.3 45.2 6.36.78 7-6 steel pipe 15.2 785 762 730 746 31.8 71.1 15.2 17.59 7-7 F1 8.2780 860 705 700 682 691 25.4 76.9 8.2 9.19 7-8 G1 Solid phase 42.7 3.8755 875 700 670 620 645 25.4 40.5 3.8 5.02 7-9 pressure 6.7 610 595 568592 15.1 64.6 6.7 9.19 welded pipe 7-10 H1 Seam 5.5 775 900 720 690 653672 15.1 64.6 5.5 9.19 welded steel pipe 7-11 I1 Solid phase 88.0 7.7750 860 675 665 642 654 42.7 51.5 7.7 9.19 pressure welded pipeCharacteristics of pipe product Crystal grain Yield Strength Tensilestrength Elongation Real pipe Area ratio of diameter YS TS (EL) Charpyductile rupture ratio Structure second phase μm MPa MPa % % * % Note0.87 632 665 44 100 F + P P:15 Invention 0.77 531 580 51 100 F + P P:15Invention 0.92 661 692 42 95 F + P + PB:22 Invention B 0.75 511 548 49100 F + P + PB:22 Invention B 0.80 688 713 37 100 F + P + PB:25Invention B 0.85 588 630 40 95 F + P + PB:25 Invention B 0.95 559 601 47100 F + C C:11 Invention 0.95 526 572 44 100 F + C C:10 Invention 0.91535 581 48 100 F + C C:10 Invention 0.88 688 736 38 95 F + M M:15Invention 0.85 463 523 46 100 F + C C:14 Invention

[0184] TABLE 14 Steel Chemical composition (wt. %) No. C Si Mn P S Al A0.43 0.32 1.53 0.008 0.003 0.015 B 0.53 0.21 0.85 0.011 0.004 0.025 C0.35 0.35 1.31 0.013 0.003 0.031 D 0.33 0.35 0.86 0.012 0.003 0.022

[0185] TABLE 15 Conditions of reduction rolling Base steel pipe Temp. ofTemp. of Outer Total Outer Crystal grain Heating starting finishing Av.rolling diameter of reduction Steel diameter diameter Structure temp.rolling rolling temp. pipe product ratio Equation (1) No. No. mm μm * °C. ° C. ° C. ° C. mm % Left side Right side 1-1 A 110 6 F + P 900 880850 865 42.7 61 6 1.9 1-2 750 730 700 715 42.7 61 6 12 1-3 750 730 700715 60.3 45 6 6.1 1-4 580 570 550 560 60.3 45 6 7.1 1-5 B 110 9 F + P700 680 650 665 42.7 61 9 13 1-6 620 610 590 600 42.7 61 9 14 1-7 C 11012 F + P 620 610 590 600 42.7 61 12 14 1-8 800 790 760 775 42.7 61 128.9 1-9 D 110 12 F + P 900 880 850 665 42.7 61 12 1.9 1-10 620 610 590600 42.7 61 12 14 Characteristic of pipe product Crystal grain YieldStrength Tensile strength Elongation Structure of Second phase diameterYS ** TS (EL) Area ratio μm MPa MPa % * % Note 7.5 504 641 37 P 65Comparative 1.0 624 721 39 C 60 Invention 4.5 540 641 35 C, P 60Comparative 1.5 685 773 37 C 60 Invention 1.5 660 759 40 C 65 Invention1.0 667 782 38 C 65 Invention 1.5 610 700 40 C 40 Invention 8.0 520 61637 C, P 40 Comparative 15 444 563 42 P 40 Comparative 1.5 553 633 43 C35 Invention

[0186] TABLE 16 Steel Chemical composition (wt. %) No. C Si Mn P S Al NCu Ni Cr Mo V Nb Ti B Ca REM O E 0.45 0.25 0.81 0.009 0.004 0.015 0.00280.15 0.20 0.12 0.08 — — — — — — 0.0023 F 0.36 0.26 0.97 0.008 0.0030.021 0.0032 — — — — 0.08 0.02 0.02 0.009 — — 0.0019 G 0.48 0.25 0.780.014 0.006 0.018 0.0035 — — — — — — — — 0.002 0.004 0.0023 H 0.35 0.251.35 0.012 0.002 0.015 0.0036 0.12 0.10 0.10 0.05 0.05 0.01 0.01 0.0010.002 — 0.0022 I 0.33 0.15 0.51 0.013 0.004 0.028 0.0043 0.15 0.20 — — —0.01 0.01 — — — 0.0025 J 0.32 0.15 0.53 0.011 0.003 0.036 0.0039 — — —0.20 0.10 — — — — — 0.0021 K 0.09 0.02 0.73 0.011 0.003 0.032 0.0036 — —— — — — — — — — 0.0025 L 0.08 0.21 0.58 0.016 0.004 0.029 0.0045 — — — —— 0.01 0.01 — — — 0.0019

[0187] TABLE 17 Conditions of reduction rolling Base steel pipe Temp. ofTemp. of Outer Total Outer Crystal grain Heating starting finishing Av.rolling diameter of reduction Steel diameter diameter Structure temp.rolling rolling temp. pipe product ratio Equation (1) No. No. mm μm * °C. ° C. ° C. ° C. mm % Left side Right side 2-1 E 110 11 F + P 670 660630 645 42.7 61 11 13.6 2-2 F 7 7 2-3 G 10 10 2-4 H 8 8 2-5 I 11 11 2-6J 10 10 2-7 K 12 12 2-8 L 11 11 Characteristic of pipe product Crystalgrain Yield Strength Tensile strength Elongation Structure of Secondphase diameter YS ** TS (EL) Area ratio μm MPa MPa % * % Note 1.5 659761 39 C 65 Invention 1.5 667 753 40 45 Invention 1.5 623 739 40 65Invention 1.0 701 796 35 45 Invention 1.5 603 678 42 40 Invention 1.5622 708 41 35 Invention 2.5 469 539 45 11 Comparative 2.0 446 530 43 8Comparative

APPLICABILITY IN INDUSTRY

[0188] In accordance with the present invention, high strength steelpipes having excellent ductility and impact resistance properties can beobtained with high productivity and by a simple process. Thus, thepresent invention extends the application field of steel pipes and istherefore particularly effective in the industry. Furthermore, thepresent invention reduces the use of alloy elements and enables low costproduction of high-strength high-ductility steel pipes improved infatigue resistance properties, or high-strength high-toughness steelpipes for use in line pipes improved in stress corrosion crackresistance. Moreover, a high strength steel material containing superfine crystal grains 1 μm or less in size is produced with superior intoughness and ductility, thereby expanding the use of steel materials.

[0189] Also available easily and without applying intermediate annealingis a steel material containing super fine crystal grains 2 μm or less insize, which yields a tensile strength of 600 MPa or more, and excellenttoughness and ductility.

What is claimed is:
 1. A super fine granular steel pipe with highcollision impact property and high workability having a compositioncontaining, by weight, 0.005 to 0.3%C, 0.01 to 3.0%Si, 0.01 to 2.0%Mn,0.001 to 0.10%Al, and balance Fe with unavoidable impurities, and across section perpendicular to a longitudinal direction of the steelpipe after reducing contains super fine grains of a ferrite having anaverage crystal grain size of 3 μm or less, and an absorbed energy up toa strain rate of 30% by performing high speed tensile tests at a strainrate of 2000s−1 is 202MJ/m³ or more, which is obtained in a method forproducing a steel pipe, comprising heating or soaking a base steel pipehaving an outer diameter of ODi (mm) and having ferrite grains with anaverage crystal diameter of di (μm) in the cross section perpendicularto the longitudinal direction of the steel pipe, and then applyingreducing at an average rolling temperature of θm(° C.) and a totalreduction ratio Tred(%) to obtain a product pipe having an outerdiameter of ODf (mm), wherein, said reducing comprises performing it ina temperature range of 400° C. or more but not more than the heating orsoaking temperature, and in such a manner that said average crystaldiameter of di (μm), said average rolling temperature of θm(° C.), andsaid total reduction ratio Tred (%) are in a relation satisfyingequation (1) as follows:di≦(2.65−0.003×θm)×10^(((0.008+θm/50000)×Tred))  (1) wherein, direpresents the average crystal diameter of the base steel pipe (μm); θmrepresents the average rolling temperature (° C.) (=(θi+θf)/2, whereinθi is a temperature of starting rolling (° C.), and θf is a temperatureof finishing rolling (° C.)); and Tred represents a total reductionratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of aproduct pipe (mm)).
 2. A super fine granular steel pipe as claimed inclaim 1, further containing one or more selected from a group consistingof 1% or less of Cu, 2% or less of Ni, 2% or less of Cr, 1% or less ofMo, or furthermore one or more selected from a group consisting of 0.1%or less of Nb, 0.5% or less of V, 0.2% or less of Ti, 0.005% or less ofB, or furthermore one or more selected from a group consisting of 0.02%or less or REM, 0.01% or less of Ca.
 3. A super fine granular steel pipewith high resistance against sulfide stress corrosion crack and highworkability having a composition containing, by weight, 0.005 to 0.1%C,0.01 to 0.5%Si, 0.01 to 1.8%Mn, 0.001 to 0.10%Al, and balance Fe withunavoidable impurities, and a cross section perpendicular to alongitudinal direction of the steel pipe after reducing contains superfine grains of a ferrite having an average crystal grain size of 3 μm orless, and in a test that a tensile stress corresponding to 120% of yieldstrength is applied to a C-ring test specimen in an NACE bath, no cracksgenerate during a test period of 200 hr, which is obtained in a methodfor producing a steel pipe, comprising heating or soaking a base steelpipe having an outer diameter of ODi (mm) and having ferrite grains withan average crystal diameter of di (μm) in the cross sectionperpendicular to the longitudinal direction of the steel pipe, and thenapplying reducing at an average rolling temperature of θm(° C.) and atotal reduction ratio Tred(%) to obtain a product pipe having an outerdiameter of ODf (mm), wherein, said reducing comprises performing it ina temperature range of 400° C. or more but not more than the heating orsoaking temperature, and in such a manner that said average crystaldiameter of di (μm), said average rolling temperature of θm(° C.), andsaid total reduction ratio Tred (%) are in a relation satisfyingequation (1) as follows:di≦(2.65−0.003×θm)×10^(((0.008+θm/50000)×Tred))  (1) wherein, direpresents the average crystal diameter of the base steel pipe (μm); θmrepresents the average rolling temperature (° C.) (=(θi+θf)/2, whereinθi is a temperature of starting rolling (° C.), and θf is a temperatureof finishing rolling (° C.)); and Tred represents a total reductionratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of aproduct pipe (mm)).
 4. A super fine granular steel pipe as claimed inclaim 3, further containing one or more selected from a group consistingof 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less of Cr, 0.5% orless of Mo, or furthermore one or more selected from a group consistingof 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti, 0.004% orless of B, or furthermore one or more selected from a group consistingof 0.02% or less or REM, 0.01% or less of Ca.
 5. A super fine granularsteel pipe with high fatigue resistance property and high workabilityhaving a composition containing, by weight, 0.06 to 0.30%C, 0.01 to1.5%Si, 0.01 to 2.0%Mn, 0.001 to 0.10%Al, and balance Fe withunavoidable impurities, and a cross section perpendicular to alongitudinal direction of the steel pipe after reducing contains superfine grains of a ferrite having an average crystal grain size of 3 μm orless, and a fatigue strength at a load stress for 10⁶ endurance cyclesis not less than 220 Mpa in a cantilever type oscillation fatigue test,which is obtained in a method for producing a steel pipe, comprisingheating or soaking a base steel pipe having an outer diameter of ODi(mm) and having ferrite grains with an average crystal diameter of di(μm) in the cross section perpendicular to the longitudinal direction ofthe steel pipe, and then applying reducing at an average rollingtemperature of θm(° C.) and a total reduction ratio Tred(%) to obtain aproduct pipe having an outer diameter of ODf (mm), wherein, saidreducing comprises performing it in a temperature range of 400° C. ormore but not more than the heating or soaking temperature, and in such amanner that said average crystal diameter of di (μm), said averagerolling temperature of θm(° C.), and said total reduction ratio Tred (%)are in a relation satisfying equation (1) as follows:di≦(2.65−0.003×θm)×10^(((0.008+θm/)50000)×Tred)  (1) wherein, direpresents the average crystal diameter of the base steel pipe (μm); θmrepresents the average rolling temperature (° C.) (=(θi+θf)/2, whereinθi is a temperature of starting rolling (° C.), and θf is a temperatureof finishing rolling (° C.)); and Tred represents a total reductionratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of aproduct pipe (mm)).
 6. A super fine granular high carbon steel pipe asclaimed in claim 5, further containing one or more selected from a groupconsisting of 1% or less of Cu, 2% or less of Ni, 2% or less of Cr, 1%or less of Mo, or furthermore one or more selected from a groupconsisting of 0.1% or less of Nb, 0.5% or less of V, 0.2% or less of Ti,0.005% or less of B, or furthermore one or more selected from a groupconsisting of 0.02% or less or REM, 0.01% or less of Ca.